Photonic switches play very important roles in advanced technologies such as telecommunications, data communications, computer interconnects, optical computing and optical signal processing. A large variety of optical switches have been developed for these applications, including electro-optic, acoustic-optic, magneto-optic and thermo-optic switches. These devices can switch optical signals from one pathway to another within a few fento-seconds to a few milliseconds depending on the switching mechanisms. Among these varieties, the electro-optic switch remains the most widely researched and used.
An electro-optic switch typically has at least one input waveguide and at least two output waveguides for carrying an optical signal. Waveguides are usually slabs, strips, or cylinders of a dielectric material or materials in "core" regions of the switch, and the waveguides are surrounded by another dielectric material ("cladding" located in cladding regions of the switch) having a lower refractive index than the material used to make its neighboring core. An optical signal enters an electro-optic switch through an inlet waveguide, and the optical signal leaves the switch from one or more output waveguides. In the conventional electro-optic switch, the cores are made of a material that changes refractive index when the material is exposed to an electric field that is created by a voltage applied to electrodes near the material. A desired amount of the power of the optical signal input into the switch is distributed to each of the exit points of the output waveguides by e.g. changing the speed at which the optical signal is transmitted through one or more waveguides within the switch.
There are at least six classifications of electro-optic switches that utilize an electro-optic material within the switch. The first classification is an interferometric-based switch such as the Mach-Zehnder switch formed in crystalline LiNbO.sub.3 and illustrated in FIG. 1A. Typically, two cores are fabricated in the switch by diffusing approximately equal amounts of titanium into the crystal. The two cores are thus made of the same electro-optic material having a refractive index n.sub.1, while the cladding surrounding the cores is made of the electro-optic crystal having a lower refractive index n.sub.2. In an interferometric-based switch, two coupled waveguides 110 and 120 separate along paths 130 and 140 so that they are spaced from each other by a sufficiently large distance that no evanescent coupling occurs between the waveguides. The optical signal splits between the two waveguides, and a portion of the signal travels through each waveguide. The waveguides are made of an electro-optic material, and the speed at which the optical signal travels through the electro-optic material in one waveguide changes in proportion to a change in an applied electric field to that waveguide created by electrodes 150 and 160. Light travels more slowly through the waveguide that has the higher refractive index, and consequently, the phase of the light traveling through that waveguide is shifted from the phase of the light traveling through the other waveguide. The waveguides subsequently approach each other within the switch and travel parallel to one another for a sufficient distance that evanescent coupling occurs between the waveguides before the optical signal is output through exit portions 170 and 180 of these waveguides. When the two light signals are recombined in the evanescently-coupled waveguides of the switch, the two light signals add together and are emitted from the exit portions of the waveguides depending on the magnitude of phase shift between the two light signals. If the phase shift is 0.degree. or 180.degree., a beam of light is emitted from each waveguide, and the beams of light have equal power. If the phase shift is 90.degree., the beam is only transmitted from one of the waveguides, and if the phase shift is 270.degree., the beam is only transmitted from the other waveguide. Other values for phase shift provide two light beams from the waveguides of unequal power. Thus, in the Mach-Zehnder switch, the power of the output signals from each of the waveguides varies as the electric field applied to the electro-optic material varies. FIG. 1A illustrates a typical Mach-Zehnder switch, and FIG. 1B shows how the power of the signal from each waveguide of the switch varies as a function of the voltage used to apply an electric field to the electro-optic material.
A second type of electro-optic switch is the .DELTA..beta. directional coupler illustrated in FIG. 2A. The .DELTA..beta. directional coupler is usually made in an electro-optic crystal such as LiNbO.sub.3 and has two waveguides comprised of cores 210 and 220 made of one electro-optic material and cladding of a second electro-optic material (the crystal). The two waveguides of the coupler are located close enough to each other that the two waveguides evanescently couple. The optical signal is inputted to an input end 230 of one of the waveguides, and the optical signal is outputted from either or both of the first output waveguide 240 (a physical continuation of the input waveguide in this switch) or the second output waveguide 250 to which the first waveguide is evanescently coupled. An applied electric field created by electrodes 260 and 270 changes the speed of light in the two waveguides (i.e. the value of n.sub.1 changes), and consequently, changes the coupling length of the waveguides changes. The power of the output signals from each of the waveguides varies as the electric field applied to the electro-optic material varies. FIG. 2B shows how the power of the signal from each waveguide varies as a function of the length of the coupler varies (L=2.pi./k, the coupling length). It has been noted in the literature that the .DELTA..beta. directional coupler requires a precise length in order to obtain cross-over from the input waveguide to the output waveguide to which the first waveguide is evanescently coupled. As FIG. 2B shows, the power in each waveguide follows a periodic function, and it is necessary to fabricate the switch to precise dimensions to obtain peak power of the optical signal in one of the waveguides.
A third type of electro-optic switch places an electro-optic grating near or over one of the two output waveguides and places a strip of e.g. titanium over one waveguide to suppress power transfer from mode interference between the two waveguides. In this switch, the electro-optic grating is made of an electro-optic material that changes the coupling mode between the waveguides from even to odd in order to transmit the optical signal from an end of one or the other of the output waveguides. The power of the signal from each waveguide as a function of the length of the waveguide generally follows the curve shown in FIG. 2B.
A fourth type of electro-optic switch is the Bragg deflection grating switch. In this switch, the optical signal entering the switch is deflected between one of two waveguides at a "Y" branch by an electro-optic grating at the branching point where the input waveguide splits into the two signal carrier waveguides.
Another type of electro-optic switch is the branching waveguide switch as illustrated in FIG. 3. The branching waveguide switch is a capital-"Y"-shaped switch, where the input waveguide 310 and an expansion region 320 form the base of the "Y" and the output waveguides 330 and 340 form the arms of the "Y" that join with the expansion region at equal angles. When the switch is fabricated in LiNbO.sub.3, the core "Y" structure is made of one uniform electro-optic material, and the surrounding cladding is the LiNbO.sub.3 crystal (which is also an electro-optic material). One electrode 350 is formed over part of the base of the "Y" and one branch of the "Y," and the second electrode 360 is formed on the same face of the switch and over another part of the base of the "Y" and the other branch of the "Y." The input waveguide carries an optical signal that is transmitted through the expansion region and directed into the two output waveguides. When no voltage is applied, each waveguide carries equal power (assuming the switch and electrodes are each perfectly symmetric). When a positive voltage is applied, more of the optical signal is switched into one of the waveguides. When a negative voltage is applied, more of the optical signal is switched into the other of the waveguides.
A sixth type of electro-optic switch is the total internal reflection (TIR) switch, as illustrated in FIG. 4. The TIR switch is an "X" configuration, where two input waveguides 410 and 420 join with two output waveguides 430 and 440 in a common intersection 450. The core region at the intersection of the waveguides is made of an electro-optic material, and two planar electrodes 460 and 470 are formed over the electro-optic material. One of the legs of the "X" carries an input optical signal. In the absence of an applied electric field, the optical signal has nothing to deflect it, and consequently the optical signal passes straight across from one input waveguide (410, for example) of the "X," through the intersection 450, and into the output waveguide opposite the input waveguide (in this example, 440). When a sufficient electric field is established, the refractive index of the electro-optic material increases such that the electro-optic material reflects the optical signal at the intersection instead of permitting it to pass through the intersection. The electro-optic material reflects the optical signal into the adjacent output waveguide of the "X" and thus prevents the optical signal from traveling straight through the intersection.
One switch that typically does not utilize an electro-optic material to switch power to output waveguides is a multimode star coupler or multimode interference coupler illustrated in FIG. 5. In this device, incident light from a single input waveguide 510 or from multiple input waveguides 510-514 is received in a mode-mixing region 520, where multiple light signals are combined or where a single signal is expanded into one broad optical signal. The signal is transmitted into multiple waveguides 531-534 by diffracting the optical signal from the input waveguide and reflecting each portion of the split signal multiple times on the edges of the waveguides until an essentially uniform optical signal is formed which has the width of the mode-mixing region.
Switch designs based on directional couplers such as the .DELTA..beta. reversed directional coupler have been investigated and developed to the extent that they can be integrated in a large array (e.g. 16.times.16 switch fabric). However, this type of switch (as well as the Mach-Zehnder, grating-directional, Bragg deflection, and multimode star coupler switches discussed above) has many drawbacks that are inherent in its design. The switch can only produce an output having a periodic function (rather than a step function) as the electric field controlling the output is changed. The switch is also extremely sensitive to the applied voltage. Consequently, each evanescent coupler will shift the optical signal from one waveguide to another at slightly different applied voltages because of minor production or operational variations in each waveguide, and consequently a switch produced today will switch optical signals from one waveguide to the next at a slightly different voltage from a switch that is produced a month from today.
A digital electro-optic switch was proposed by Silberberg et al. and constructed (Y. Silberberg, P. Perlmutter, & J. E. Baran, "Digital Optical Switch," Appl. Phys. Lett., Vol. 51, No. 16, pp. 1230-32 (Oct. 19, 1987)). This switch provides low sensitivity to deviations in manufacture and operation of the switch for both switching states, but this switch requires long device lengths and high switching voltages. In order to overcome these shortfalls, a new type of hybrid digital electro-optic switch has been developed.